239 research outputs found
Maximum gradient embeddings and monotone clustering
Let (X,d_X) be an n-point metric space. We show that there exists a
distribution D over non-contractive embeddings into trees f:X-->T such that for
every x in X, the expectation with respect to D of the maximum over y in X of
the ratio d_T(f(x),f(y)) / d_X(x,y) is at most C (log n)^2, where C is a
universal constant. Conversely we show that the above quadratic dependence on
log n cannot be improved in general. Such embeddings, which we call maximum
gradient embeddings, yield a framework for the design of approximation
algorithms for a wide range of clustering problems with monotone costs,
including fault-tolerant versions of k-median and facility location.Comment: 25 pages, 2 figures. Final version, minor revision of the previous
one. To appear in "Combinatorica
Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid
We present an optical mode solver for a whispering gallery resonator coupled
to an adjacent arbitrary shaped nano-particle that breaks the axial symmetry of
the resonator. Such a hybrid resonator-nanoparticle is similar to what was
recently used for bio-detection and for field enhancement. We demonstrate our
solver by parametrically studying a toroid-nanoplasmonic device and get the
optimal nano-plasmonic size for maximal enhancement. We investigate cases near
a plasmonic resonance as well as far from a plasmonic resonance. Unlike common
plasmons that typically benefit from working near their resonance, here working
far from plasmonic resonance provides comparable performance. This is because
the plasmonic resonance enhancement is accompanied by cavity quality
degradation through plasmonic absorption.Comment: Supplementary COMSOL script, see
http://www.quantumchaos.de/Media/comsol2013/Supplement_Script_for_Fig.3_Comsol_4.3a.mp
Online Makespan Minimization with Parallel Schedules
In online makespan minimization a sequence of jobs
has to be scheduled on identical parallel machines so as to minimize the
maximum completion time of any job. We investigate the problem with an
essentially new model of resource augmentation. Here, an online algorithm is
allowed to build several schedules in parallel while processing . At
the end of the scheduling process the best schedule is selected. This model can
be viewed as providing an online algorithm with extra space, which is invested
to maintain multiple solutions. The setting is of particular interest in
parallel processing environments where each processor can maintain a single or
a small set of solutions.
We develop a (4/3+\eps)-competitive algorithm, for any 0<\eps\leq 1, that
uses a number of 1/\eps^{O(\log (1/\eps))} schedules. We also give a
(1+\eps)-competitive algorithm, for any 0<\eps\leq 1, that builds a
polynomial number of (m/\eps)^{O(\log (1/\eps) / \eps)} schedules. This value
depends on but is independent of the input . The performance
guarantees are nearly best possible. We show that any algorithm that achieves a
competitiveness smaller than 4/3 must construct schedules. Our
algorithms make use of novel guessing schemes that (1) predict the optimum
makespan of a job sequence to within a factor of 1+\eps and (2)
guess the job processing times and their frequencies in . In (2) we
have to sparsify the universe of all guesses so as to reduce the number of
schedules to a constant.
The competitive ratios achieved using parallel schedules are considerably
smaller than those in the standard problem without resource augmentation
Nonlinear optics and light localization in periodic photonic lattices
We review the recent developments in the field of photonic lattices
emphasizing their unique properties for controlling linear and nonlinear
propagation of light. We draw some important links between optical lattices and
photonic crystals pointing towards practical applications in optical
communications and computing, beam shaping, and bio-sensing.Comment: to appear in Journal of Nonlinear Optical Physics & Materials (JNOPM
Exploiting disorder for perfect focusing
We demonstrate experimentally that disordered scattering can be used to
improve, rather than deteriorate, the focusing resolution of a lens. By using
wavefront shaping to compensate for scattering, light was focused to a spot as
small as one tenth of the diffraction limit of the lens. We show both
experimentally and theoretically that it is the scattering medium, rather than
the lens, that determines the width of the focus. Despite the disordered
propagation of the light, the profile of the focus was always exactly equal to
the theoretical best focus that we derived.Comment: 4 pages, 4 figure
Anomalous spectral scaling of light emission rates in low dimensional metallic nanostructures
The strength of light emission near metallic nanostructures can scale
anomalously with frequency and dimensionality. We find that light-matter
interactions in plasmonic systems confined in two dimensions (e.g., near metal
nanowires) strengthen with decreasing frequency owing to strong mode
confinement away from the surface plasmon frequency. The anomalous scaling also
applies to the modulation speed of plasmonic light sources, including lasers,
with modulation bandwidths growing at lower carrier frequencies. This allows
developing optical devices that exhibit simultaneously femto-second response
times at the nano-meter scale, even at longer wavelengths into the mid IR,
limited only by non-local effects and reversible light-matter coupling
Personalized Feedback on Staff Dose in Fluoroscopy-Guided Interventions: A New Era in Radiation Dose Monitoring
Radiation safety and protection are a key component of fluoroscopy-guided interventions. We hypothesize that providing weekly personal dose feedback will increase radiation awareness and ultimately will lead to optimized behavior. Therefore, we designed and implemented a personalized feedback of procedure and personal doses for medical staff involved in fluoroscopy-guided interventions. Medical staff (physicians and technicians, n = 27) involved in fluoroscopy-guided interventions were equipped with electronic personal dose meters (PDMs). Procedure dose data including the dose area product and effective doses from PDMs were prospectively monitored for each consecutive procedure over an 8-month period (n = 1082). A personalized feedback form was designed displaying for each staff individually the personal dose per procedure, as well as relative and cumulative doses. This study consisted of two phases: (1) 1-5th months: Staff did not receive feedback (n = 701) and (2) 6-8th months: Staff received weekly individual dose feedback (n = 381). An anonymous evaluation was performed on the feedback and occupational dose. Personalized feedback was scored valuable by 76% of the staff and increased radiation dose awareness for 71%. 57 and 52% reported an increased feeling of occupational safety and changing their behavior because of personalized feedback, respectively. For technicians, the normalized dose was significantly lower in the feedback phase compared to the prefeedback phase: [median (IQR) normalized dose (phase 1) 0.12 (0.04-0.50) A mu Sv/Gy cm(2) versus (phase 2) 0.08 (0.02-0.24) A mu Sv/Gy cm(2), p = 0.002]. Personalized dose feedback increases radiation awareness and safety and can be provided to staff involved in fluoroscopy-guided interventions
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